1. Field of the Invention
The present invention relates to an image forming apparatus and a method for correcting position displacement.
2. Description of the Related Art
A tandem type image forming apparatus forms color images by using image forming parts (imaging parts) corresponding to different colors (e.g., black (BK), cyan (C), magenta (M), yellow (Y)) and superposing toner images of each color onto paper (recording medium) or a transfer belt.
This type of image forming apparatus may face a problem of color displacement which is caused by toner images of each color deviating from a target position where the toner images are to be superposed. Therefore, with this type of image forming apparatus, position displacement is corrected by forming a position displacement correction pattern (toner mark pattern), detecting the position at which each image is formed by using a pattern detecting part (e.g., TM (Toner Mark) sensor), measuring the displacement amount (amount of displacement with respect to a target position) by referring to the toner mark pattern, and calculating the correction amount (amount of displacement to be corrected). Thereby, occurring of color displacement can be reduced and color images can be consistently formed.
However, after a substantial amount of time elapses after the correcting of position displacement, position displacement may reoccur. Therefore, the image forming apparatus is required to repeat the position displacement correcting operation. This increases the consumption amount of toner used for forming the position displacement correction patterns.
In order to the reduce the toner consumption amount, there is proposed a method of reducing the number of times for forming position displacement correction patterns or a method of reducing the size of the position displacement correction patterns (see, for example, Japanese Laid-Open Patent Publication Nos. 2000-066463 and 2005-233983).
Even in a case where multiplication error deviation having little effect on formation of color images occurs, the proposed methods form position displacement correction patterns at a center area for correcting the multiplication error deviation. Because position displacement correction patterns are formed even in the case where correction of multiplication error deviation is unnecessary, toner is wasted.